You may already know most of these, but here are some suggestions:
3 (Bands own description: dark yet uplifting, spiritual without any connection to religion.)
Björk (Great singer, obscure electronic stuff from Iceland.)
Jeff Buckley (Alternative singer-songwriter rock, greatest singer ever.)
Kate Bush (Discovered by David Gilmour. Never For Ever record is my personal favourite.)
Coldplay (The early stuff is really good. I especially like the first album.)
Guthrie Govan (Maybe the only instrumental music that doesn't bore me. Greatest guitarist of this century.)
Pink Floyd (Greatest progressive rock band ever.)
Radiohead (Check out the legendary OK Computer album, if you haven't yet.)
Stevie Wonder (Try the Innervisions record for example. Great stuff.)
The Mars Volta (Good experimental / progressive rock.)